1. Field of Invention
This invention relates generally to analog circuits, and more particularly supplying power for a vacuum fluorescent display (VFD) filament.
2. Description of Relevant Art
Vacuum fluorescent displays (VFDs) are a type of display that utilizes thermal emission of electrons from a cathode also referred to as a VFD filament (typically about 640xc2x0 C.) and phosphor excitation at a target anode to generate a color display. In contrast to a CRT, for example, the electrons are accelerated by a much lower voltage and the pixels are switched on/off by changing the polarity of the electric potential at the target anode. For example, a positively charged target will attract electrons whereas a negatively charged target will repel them. In this way, attracted electrons excite the phosphors, which thereby emit light. It is one of the advantages of a VFD that the phosphors can be patterned in any shape making VFDs suitable for displaying, among other things, icons in consumer electronics. Due to their ruggedness and high luminance, VFDs are also employed in automobile dashboard and various headup type displays.
Typically, a VFD filament requires an AC power supply source in order to create a constant brightness across its entire length. Unfortunately, however, currently available AC power supplies are deficient in any number of ways requiring, for example, an AC power input or in some cases generating high electromagnetic interference (EMI) making them unsuitable for use in many consumer electronics as well as automobile displays. An another disadvantage of currently available AC power supplies is that they require expensive circuits to shut them off when the system is not in use.
Therefore, what is desired is a system and apparatus for supplying power to a VFD filament that provides a substantially uniform display, substantially free of EMI and also an inexpensive shutdown feature.
The invention is an electrical circuit that provides power to a VFD filament without resorting to using an AC power supply. In this way, the circuit substantially reduces the electromagnetic interference (EMI) generated by conventionally arranged VFD filament power supplies.
In one embodiment, an apparatus for supplying power to a vacuum fluorescent display (VFD) filament in a vacuum fluorescent display (VFD) unit having a display anode is described. The apparatus includes a DC power supply unit, a self-oscillating stage coupled to the DC power supply having a self-oscillating stage output node connected to a first terminal of the VFD filament, and an inverter stage coupled to the DC power supply having an inverter stage input node coupled to the self-oscillating stage output node and an inverter stage output node connected to a second terminal of the VFD filament. The self-oscillating stage provides a first output voltage waveform to the first terminal, and the inverter stage provides a second output voltage waveform to the second terminal such that the first output voltage waveform and the second output voltage waveform are anti-phase such that the VFD filament provides a substantially constant average voltage difference between the anode(s) and different points of the filament.
In another embodiment, a driver circuit for supplying power to a vacuum fluorescent display (VFD) filament in a vacuum fluorescent display (VFD) unit having a display anode is disclosed. The driver circuit includes a DC power supply unit, a first power operational amplifier coupled to the DC power supply having a first power operational amplifier output node connected to a first terminal of the VFD filament, and a second power operational amplifier coupled to the DC power supply having an second power operational amplifier input node coupled to the first power operational amplifier output node and a second power operational amplifier output node connected to a second terminal of the VFD filament. The first power operational amplifier provides a first output voltage waveform to the first terminal, and the second power operational amplifier provides a second output voltage waveform to the second terminal such that the first output voltage waveform and the second output voltage waveform are in anti-phase relation such that the VFD filament provides a substantially uniform voltage differential profile in relation to the display anode. The driver circuit also includes an external clock source coupled to the first power operational amplifier arranged to control a frequency of the first output waveform and the second output waveform, and a first shape of the first output waveform and a second shape of the second output waveform.
In still another embodiment, a method of providing power to a VFD filament is described. The method includes providing a DC power supply unit, coupling a self-oscillating stage coupled to the DC power supply having a self-oscillating stage output node, connecting the self-oscillating stage output node to a first terminal of the VFD filament, coupling an inverter stage to the DC power supply having an inverter stage input node, and coupling the inverter stage input node to the self-oscillating stage output node. The method further includes coupling an inverter stage output node to a second terminal of the VFD filament such that the self-oscillating stage provides a first output voltage waveform to the first terminal and the inverter stage provides a second output voltage waveform to the second terminal such that the first output voltage waveform and the second output voltage waveform are anti-phase such that the VFD filament provides a substantially uniform voltage differential profile in relation to the display anode.
These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following descriptions of the invention and a study of the several figures of the drawing.